This invention relates to filters, and more particularly, to continuous differential filters that use gyrators or the like to simulate one or more inductance values. Most electrical systems include at least some form of an electrical filter such as a low pass, a high pass, or a bandpass filter. These filters are typically implemented using a combination of discrete components such as resistors, inductors and/or capacitors. In some technologies, such as integrated circuit and some printed circuit board technologies, inductors cannot readily be produced. To overcome this limitation, gyrators have been developed. Gyrators simulate an inductance using, for example, only active elements such as transistors and a capacitance load. Gyrators thus help eliminate the need for conventional physical inductors (e.g., coils).
Gyrators often have an input impedance that is proportional to the load admittance. Therefore, when a gyrator is loaded with a capacitance, the input impedance behaves like an inductance. Some prior art gyrator circuits are described in, for example, U.S. Pat. No. 3,643,183 to Geffe, U.S. Pat. No. 3,715,693 to Fletcher et al., U.S. Pat. No. 3,758,885 to Voorman et al., and U.S. Pat. No. 4,812,785 to Pauker.
In integrated circuit technologies, the load capacitance used by the gyrator is typically formed using a gate oxide type capacitor. Gate oxide capacitors include a gate oxide layer cladded by the substrate layer and the polysilicon gate layer. The capacitance value of a gate oxide capacitor is primarily dictated by the area of the polysilicon gate region. Even though the gate oxide layer is relatively thin, the amount of capacitance that can be generated per unit area is relatively small. Therefore, to generate an adequate capacitance value for many filter applications, the area of the gate oxide capacitor must be relatively large, which can significantly reduce the overall density, reliability and yield of the integrated circuit (IC).
In many integrated circuit processes, the gate oxide layer may be susceptible to pin holing, wherein one or more pinhole defects in the gate oxide effectively short the substrate to the polysilicon gate layer. The probability of having a pin hole in any given circuit is typically dependent on the total gate oxide area in the circuit. Thus, when large gate oxide capacitors are used, the chance of having one or more pin holes in the circuit increases, and the overall yield of the circuit decreases. Therefore, it would be desirable to produce a gyrator based filter circuit that minimizes the total area of the gate oxide capacitors.
Gyrator filters are also often only adapted to accept and filter single ended input signals. For some applications, it would be desirable to provide a gyrator based filter that is adapted to accept and filter differential input signals. Differential input signals typically provide an improved signal-to-noise ratio relative to a single ended input signal, and can increase the dynamic range of the circuit. This is particularly important for low power applications where the peak-to-peak signal level is relatively small, and when the gyrator circuit is integrated on a single IC along with other circuits that generate substantial substrate and power supply noise. Therefore, it would also be desirable to provide a gyrator based filter that is adapted for receiving and filtering a differential input signal.